The present invention relates to a method for intensifying the operation of a Yankee press in a soft-tissue paper machine, in which a web is pressed in one or several nips formed between a Yankee cylinder having an overlying Yankee hood and a press roll. The web is dried on the Yankee cylinder by blowing hot air introduced through the Yankee hood against the web. In the method in accordance with the invention, a blow device is used which blows hot and humid air against the web before the web enters into the Yankee press.
The invention also relates to a device for carrying out the method in accordance with the invention to intensify the operation of a Yankee press in a soft-tissue machine, wherein the Yankee press comprises a Yankee cylinder having an associated Yankee hood and one or several press rolls forming nip(s) with the Yankee cylinder. The device comprises a blow device having a blow chamber, an exhaust chamber, and blow nozzles for blowing hot and humid air against the web and exhaust-air ducts for removing the air.
In the manufacture of paper, a fibrous web is formed in the wire part of the paper machine on an endless wire or between two wires. Thereafter, water is drained out of the fiber web in several stages, the first of which is usually achieved by passing the fibrous web over a wire zone provided with suction boxes and also possibly suction rolls. In the wire zone, the suction boxes and/or suction rolls draw water out of the web by means of negative pressure. The fibrous web is then passed to the press section where water is removed out of the web by pressing the web between press rolls. In the press section, the web is usually supported on a felt so that water is also removed out of the felt at the same time, e.g., by making use of felt absorbers operating with negative pressure. After the press section, the web is usually dried by evaporation in the dryer section of the paper machine wherein, e.g., the web runs over a Yankee cylinder provided with a Yankee hood.
In view of the energy economy and desired efficiency of the paper machine, dewatering of the web by mechanical pressing in the press section of a paper machine is substantially more advantageous than drying that takes place by evaporation in the dryer section. The result achieved by drying in the press section also affects the result of the drying in the dryer section.
When water is removed out of the paper web, the web is generally cooled so that the viscosity of the water contained in the fibrous web becomes higher and dewatering of the web in subsequent pressing steps becomes more difficult. Cooling of the web also results in a higher energy requirement in the dryer section. For this reason, attempts have been made to find solutions for heating the web to a suitable temperature so as to lower the viscosity of the water present in the web and, thus, to facilitate the pressing of water out of the web.
In paper machines, steam cylinders, steam boxes, and electric/gas infra-dryers have been used for heating the web. A common feature of these prior art devices is the considerable energy requirements are needed to obtain an adequate temperature of the web.
As is well known in the art, steam cylinders are used in conventional paper machines in which the speed of the paper machine does not impose restrictions for their use. The field of application of steam boxes has been mainly for profiling of the web. In the prior art devices based on steam boxes, an increased capacity of a paper machine has been obtained indirectly by means of a more uniform moisture profile, whereby so-called over-drying can be substantially avoided. However, by means of a steam box, an increase in the capacity of a paper machine cannot be obtained if the profile is uniform. Steam boxes have also been used in multi-nip "Sym-Press" (trade mark) presses between the press nips in order to heat the web.
In the environment of a press section of a paper machine, the use of electric/gas infra-dryers for heating of the web has proved to be quite problematic, because these apparatuses are contaminated rapidly in this area, which deteriorates their efficiency. The contamination is usually caused by loose particles.
A conventional Yankee press comprises a large-diameter Yankee cylinder that is heated from inside and a Yankee hood that consists of one or several blocks and is formed over a part of the circumference of the cylinder. Warm air is blown through the hood against the web in order to dry the web. Before the hood section, a Yankee press usually comprises one or several press rolls which form(s) a press nip(s) together with the Yankee cylinder. In these press nips, water is removed out of the web by pressing.
In the prior art Yankee presses, it is a drawback that it is not always possible to raise the efficiency of the press nips to a sufficiently high level because the web has time to be cooled in the preceding steps so that the viscosity of the water becomes higher and the removal of water in the press nips is not so efficient. This has also resulted in a higher energy requirement for adequately drying the web in view of the drying capacities of the Yankee cylinders and hoods.
Devices and methods are also known in the prior art which are based on air blowing. In these devices and methods, warm and humid air is blown against the web so as to heat the web and to thereby intensify both the pressing and the drying steps. However, in these prior art devices, it is a drawback that relatively low values of humidity and temperature of the air have been used so that the efficiency has been comparatively low.
A particular prior art device based on air blowing in order to intensify the dewatering of a web in a paper, board or pulp machine is described in Finnish Patent Application No. FI 54,956. In this prior art device, warm and humid exhaust air from the dryer section or, as an alternative, warm and humid exhaust air from the pressurized process of a thermo-mechanical pulping plant, has been used to intensify the dewatering of the web.